Since I started working with Singularity in early 2016, I’ve wanted to create an orchestration tool for it. I knew it would be a long time coming, because not only did we need to be able to stop and start containers, we also needed to be able to (akin to Docker) create a bridge network (conceptually similar to a router) on the host, and then programmatically query for their process ids.

In the last week or so, I decided that it was time! Today I want to share you an Alpha release of singularity-compose” (yes it is intentionally named to mirror docker-compose) and it’s also implemented in Python like it’s counterpart. This was also an easy choice because Singularity Python already has nice handles to work with instances and Singularity from Python.

Why an Alpha Release?

While most of the ducks are in a row, there is a networking issue that makes it impossible for instances on Linux hosts that have used Docker (like mine) to see one another. But actually, this exact bug ultimately meant that I learned a ton about networking, and that is exactly what I want to share with you today. If you want an overview of singularity-compose (alpha), there is a beautiful guide for you here with a working (single container) example. For those interested, here is a quick demo of the single container example:

The beginning has a weird echo (I don’t use anything beyond the mic on my laptop, so there’s that) and the color is also completely off (meh?). I’ll update you when the multi-container example is ready to go, meaning the networking issue linked above is closed. For now, let’s jump into some basics that I learned about networking.


Mind you I’m still rather green, and I’m describing these components in a way that makes sense to me. I want to say a big dinosaur thank you to my colleagues @griznog, @dwd and @cclerget (from Sylabs). for the great fun we had yesterday starting to debug this issue.


If you type hostname on most linux systems, you see something like this:

$ hostname

It’s actually both written into a file “/etc/hostname”:

$ cat /etc/hostname 

and an entry in “/etc/hosts”

$ cat /etc/hosts	localhost	vanessa-ThinkPad-T460s

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Without getting into the gritty details, it’s the name of your computer. This was my starting point when I started working with instances yesterday. I knew that, within each container instance, given that the instance could “see” the addresses of the others, I’d want to give them a name for the user. So instead of needing to hard code some ephemeral address into some dependency file in a container, you can write the name of the instance (like “app” or “nginx”). Let’s say I created instances named “app” and “nginx.” They might have entries like this:	app	nginx	localhost

And then in some nginx.conf (a nginx configuration file) I could create a block that references “app,” fully knowing that the actual address might change (but the name will stay constant).

upstream django {
    server app:3031;      # web port socket

Bridge Networks

Okay, so great! We now (conceptually) know that we want to bind a hosts file to each instance so that it can associate an ip address with a name.

But wait, how do we know if an instance can see another’s address?

I thought about this. What first came to mind was the idea of a bridge. I knew that Docker was creating bridges in order for containers to talk to one another. I read in the Singularity docs that a bridge is created by default for Singularity 3.0 and over. Great!

Um, but where is it?

I found a command called “brctl” (bridge control!) that will list bridges on your host:

$ brctl show
bridge name	bridge id		STP enabled	interfaces
docker0		8000.0242e77953e2	no		
sbr0		8000.2280256f1d10	no		

I was really quite overjoyed to see docker0 there, because I would have expected that. I got more excited when I saw “sbr0” because I hoped that it meant “Singularity Bridge” and its existence was an indication that the bridge was created as expected. I then realized that I could of course look in the singularity source code to confirm this. I found a configuration file that creates it! And actually, these are stored locally:

$ ls /usr/local/etc/singularity/network/
00_bridge.conflist  10_ptp.conflist  20_ipvlan.conflist  30_macvlan.conflist

And that confirmed it. And guess what, you can easily create your own. I thought about doing this for singularity-compose, but if it’s being installed by a user (without permission to write to that particular folder) it would be a hassle when it failed, and I’d have to provide another workaround anyway. So instead I decided to develop using the default bridge. This means that on a single host, all container instances (from a singularity-compose application and others started at random, for example) would see one another. This wouldn’t be desired for production given a shared host, but for the average user to orchestrate instances it will do. I suspect I’ll add an argument at some point to create and use a custom bridge, for the advanced user.

What about the instances?

The next veriication I needed was that when I created instances, they were added to it. This would mean that their addresses (assigned in the address space of the bridge) would be viewable to one another. All I had to do in this case was start a group of containers (with singularity-compose). If you run it with --debug it will show you the commands that are run to start the instances. Here is one (abridged) example (networking pun!):

$ singularity instance start \
  --bind /home/vanessa/Documents/Dropbox/Code/singularity/singularity-compose-simple/etc.hosts:/etc/hosts \
  --net --network-args "portmap=80:80/tcp" --network-args "IP=" \
  --hostname app \
  --writable-tmpfs app.sif app

Notice that we provide “–net” to indicate that we want to set up networking, following by other network arguments. We could have even defined a specific kind (or custom) network with “–network”. I’ll also quickly tell you that, based on knowing the sbr0 bridge used “”, I was able to pre-emptively generate the addresses for the containers, write them into host files to bind (see /etc/hosts above) and assign the address with “–network-args”. But check this out - there is an entire project to provide plugins for Container networking! It’s referred to as “CNI” and if you look in the Singularity source code, it’s being used. This is how we told Singularity to expose port 80:

--network-args "portmap=80:80/tcp"

And we could also specify to create a bridge within the container! I actually did this first, and then realized I didn’t need to. If I had created separate bridges, I’d need to connect them with a fabric. I think maybe we do this with flannel? I’m not sure. I need to read more about it.

And a quick Python trick - since we know the address of the bridge sbr0, we can use the ipaddress module to generate addresses for us. I wrote a function to generate a lookup table of ip address by name, to pass around:

def get_ip_lookup(names, bridge=""):
    '''based on a bridge address that can serve other addresses (akin to
       a router, metaphorically, generate a pre-determined address for
       each container.

       names: a list of names of instances to generate addresses for. 
       bridge: the bridge address to derive them for.
    host_iter = IPv4Network(bridge).hosts()
    lookup = {}
    # Don't include the gateway

    for name in names:
        lookup[name] = str(next(host_iter))

    return lookup

So once I had run singularity-compose to bring up my instances, lo and behold - the interfaces appeared!

$ brctl show sbr0
bridge name	bridge id		STP enabled	interfaces
sbr0		8000.2280256f1d10	no		veth2cdea445

At this point, we hit the bug that I couldn’t ping one instance ip from the other, and I moved on to creating the working example with a single container in the docs. That worked, so I finished up the example, these docs, and I’ll report back when the networking bug is fixed and we can do examples with multiple containers.


We’ve walked through a simple example of showing how singularity-compose can help you to orchestrate container instances. I’d like to close with some wisdom from my colleague @griznog, who has so many good quotes, he really should have his own website just to house them. To give you some context, he had compared “/etc/hosts” with a phone book, and was describing the meaning of “localohost” (and why I couldn’t change it):

The takehome there is that you can’t talk to other people on localhost, it’s only for the voices in your head. Where “your” refers to each phone. /etc/hosts is a convenience, only needed if you can’t remember everyone else’s number.

It’s definitely a good idea to not let any others in on those voices :) Thanks everyone, and if you’d like to follow singularity-compose or ask a question, you know what to do.

Suggested Citation:
Sochat, Vanessa. "Singularity Compose: Alpha." @vsoch (blog), 20 Jun 2019, (accessed 16 Apr 24).